This invention relates to elastic substantially linear olefin polymers having improved processability. e.g., low susceptibility to melt fracture, even under high shear stress extrusion conditions. Methods of manufacturing these polymers are also disclosed.
Molecular weight distribution (MWD), or polydispersity, is a well known variable in polymers. The molecular weight distribution, sometimes described as the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (i.e., Mw/Mn) can be measured directly, e.g., by gel permeation chromatography techniques, or more routinely, by measuring I10/I2 ratio, as described in ASTM D-1238. For linear polyolefins, especially linear polyethylene, it is well known that as Mw/Mn increases, I10I2 also increases.
John Dealy in xe2x80x9cMelt Rheology and Its Role in Plastics Processingxe2x80x9d (Van Nostrand Reinhold, 1990) page 597 discloses that ASTM D-1238 is employed with different loads in order to obtain an estimate of the shear rate dependence of melt viscosity, which is sensitive to weight average molecular weight (Mw) and number average molecular weight (Mn).
Bersted in Journal of Applied Polymer Science Vol. 19, page 2167-2177 (1975) theorized the relationship between molecular weight distribution and steady shear melt viscosity for linear polymer systems. He also showed that the broader MWD material exhibits a higher shear rate or shear stress dependency.
Ramamurthy in Journal of Rheology, 30(2), 337-357 (1986), and Moynihan, Baird and Ramanathan in Journal of Non-Newtonian Fluid Mechanics, 36, 255-263 (1990), both disclose that the onset of sharkskin (i.e., melt fracture) for linear low density polyethylene (LLDPE) occurs at an apparent shear stress of 1-1.4xc3x97106 dyne/cm2, which was observed to be coincident with the change in slope of the flow curve. Ramamurthy also discloses that the onset of surface melt fracture or of gross melt fracture for high pressure low density polyethylene (HP-LDPE) occurs at an apparent shear stress of about 0.13 MPa (1.3xc3x97106 dynes/cm2).
Kalika and Denn in Journal of Rheology, 31, 815-834 (1987) confirmed the surface defects or sharkskin phenomena for LLDPE, but the results of their work determined a critical shear stress of 2.3xc3x97106 dyne/cm2, significantly higher than that found by Ramamurthy and Moynihan et al.
International Patent Application (Publication No. WO 90/03414) published Apr. 5, 1990, discloses linear ethylene interpolymer blends with narrow molecular weight distribution and narrow short chain branching distributions (SCBDs). The melt processibility of the interpolymer blends is controlled by blending different molecular weight interpolymers having different narrow molecular height distributions and different SCBDs.
Exxon Chemical Company, in the Preprints of Polyolefins VII International Conference, page 45-66, Feb. 24-27, 1991, disclose that the narrow molecular weight distribution (NMWD) resins produced by their EXXPOL(trademark) technology have higher melt viscosity and lower melt strength than conventional Ziegler resins at the same melt index. In a recent publication, Exxon Chemical Company has also taught that NMWD polymers made using a single site catalyst create the potential for melt fracture (xe2x80x9cNew Specialty Linear Polymers (SLP) For Power Cables,xe2x80x9d by Monica Hendewerk and Lawrence Spenadel, presented at IEEE meeting in Dallas, Tex., September, 1991).
Previously known narrow molecular weight distribution linear polymers disadvantageously possessed low shear sensitivity or low I10/I2 value, which limits the extrudability of such polymers. Additionally, such polymers possessed low melt elasticity, causing problems in melt fabrication such as film forming processes or blow molding processes (e.g., sustaining a bubble in the blown film process, or sag in the blow molding process etc.). Finally, such resins also experienced melt fracture surface properties at relatively low extrusion rates thereby processing unacceptably.
We have now discovered a new family of substantially linear olefin polymers which have many improved properties and a method of their manufacture. The substantially linear olefin polymers have (1) high melt elasticity and, (2) relatively narrow molecular weight distributions with exceptionally good processibility while maintaining good mechanical properties and (3) they do not melt fracture over a broad range of shear stress conditions. These properties are obtained without benefit of specific processing additives. The new polymers can be successfully prepared in a continuous polymerization process using constrained geometry catalyst technology, especially when polymerized utilizing solution process technology.
The improved properties of the polymers include improved melt elasticity and processability in thermal forming processes such as extrusion, blowing film, injection molding and blow-molding.
Substantially linear polymers made according to the present invention have the following novel properties:
a) a melt flow ratio, I10/I2, xe2x89xa75.63,
b) a molecular weight distribution, Mw/Hn, defined by the equation:
Mw/Mnxe2x89xa6(I10/I2)xe2x88x924.63, and
c) a critical shear stress at onset of gross melt fracture of greater than about 4xc3x97106 dyne/cm2.